Microfluidics is a rapidly expanding field concerned with the manipulation and precise control of fluids on a small scale, often dealing with sub-microlitre volumes. There is growing interest in its application to chemical or biochemical assay and synthesis, both in research and production, and applied to healthcare diagnostics (“lab-on-a-chip”). In the latter case, the small nature of such devices allows rapid testing at point of need using much smaller clinical sample volumes than for traditional lab-based testing.
A microfluidic device can be identified by the fact that it has one or more channels (or more generally gaps) with at least one dimension less than 1 millimetre (mm). Common fluids used in microfluidic devices include whole blood samples, bacterial cell suspensions, protein or antibody solutions and various buffers. Microfluidic devices can be used to obtain a variety of interesting measurements including molecular diffusion coefficients, fluid viscosity, pH, chemical binding coefficients and enzyme reaction kinetics. Other applications for microfluidic devices include capillary electrophoresis, isoelectric focusing, immunoassays, enzymatic assays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, PCR amplification, DNA analysis, cell manipulation, cell separation, cell patterning and chemical gradient formation. Many of these applications have utility for clinical diagnostics.
Many techniques are known for the manipulation of fluids on the sub-millimetre scale, characterised principally by laminar flow and dominance of surface forces over bulk forces. Most fall into the category of continuous flow systems, often employing cumbersome external pipework and pumps. Systems employing discrete droplets instead have the advantage of greater flexibility of function.
Electro-wetting on dielectric (EWOD) is a well-known technique for manipulating discrete droplets of fluid by application of an electric field. It is thus a candidate technology for microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?” (R. B. Fair, Microfluid Nanofluid (2007) 3:245-281). This review notes that methods for introducing fluids into the EWOD device are not discussed at length in the literature. It should be noted that this technology employs the use of hydrophobic internal surfaces. In general, therefore, it is energetically unfavourable for aqueous fluids to fill into such a device from outside by capillary action alone. Further, this may still be true when a voltage is applied and the device is in an actuated state. Capillary filling of non-polar fluids (e.g. oil) may be energetically favourable due to the lower surface tension at the liquid-solid interface.
An Electrowetting-on-Dielectric (EWOD) device or other microfluidic device can be used to meter and mix fluids together, either to perform an assay within the device (this requires some kind of read-out, e.g. optical, electrical), or prepare a sample to be analysed elsewhere using a different kind of reader. In the latter case, one or more droplets must be ‘extracted’ from the device. Currently this is most often done by pipette through a hole in the upper substrate. However, while this extraction method is suitable for use by a trained worker in laboratory conditions it is not reliable in general use.
U.S. Pat. No. 9,132,400 proposes a method of dispensing liquid for use in biological analysis that comprises positioning liquid to be dispensed through electrowetting. This is illustrated in FIG. 2. A loader has an upper substrate 25 covered by a hydrophobic insulator layer 24 with electrical pads 45 disposed between the substrate 25 and the layer 24, and has a lower substrate 60 covered by a hydrophobic insulator layer 74 with an electrode 75 disposed between the lower substrate 60 and the layer 74. The substrates are spaced from one another and sealed with a seal 65. An input port 40 is provided in the upper substrate 25 to permit loading of fluid 200 into the chamber 70 of the loader. At least one opening 50 is provided in the lower substrate 25. U.S. Pat. No. 9,132,400 proposes that the electrical charge of the electrical pads 45 and the electrode 75 may be controlled so as to move liquid in a single direction within the chamber to be positioned in alignment with an opening 50 for dispensing therethrough as indicated by droplet 201.